The present invention relates to a method for burning a carbonaceous material such as coal. More particularly, the invention relates to a method for burning a pulverized carbonaceous material in the form of a waterbased slurry to reduce the emission of particulates and sulfur compounds in the combustion gases.
In the combustion of carbonaceous materials containing sulfur and ash, such as coal, the gases of combustion may contain sulfur compounds representing a major portion of the sulfur found in the material, as well as ash particulates. Since these sulfur compounds and particulates may constitute significant environmental hazards, much work has been devoted to the development of methods for preventing formation of these substances or cleansing them from the combustion gases.
One method for the combustion of a carbonaceous material such as coal in a manner which is aimed at a reduction in the sulfur content of the combustion gases is described in U.S. Pat. No. 3,540,387 of McLaren et al. According to this method, the carbonaceous material and a specified amount of a carbonate, such as calcium carbonate, are fed into a fluidized bed of ash, where the carbonaceous material is burned at a temperature within the range of 700.degree.-900.degree. C. The practice of this method reportedly reduces the sulfur content of the gases of combustion because the presence of the carbonate leads to ready retention of the sulfur in the ash produced in the combustion process.
A method for operating a powdered coal combustion furnace in such a way as to control the form in which ash is produced therein, in order to minimize problems associated with disposal of the ash, is described in U.S. Pat. No. 1,955,574 of Benner et al. According to this method, a reagent is added during combustion which alters the melting point of the coal ash so that a substantially uniform temperature differential is maintained between the melting point of the ash and the flame temperature of combustion. The method of Benner et al. recites that the melting point of the ash may generally be raised by the addition of sand or a non-ferruginous clay to the coal, and that the addition of lime or soda to the coal will lower its ash melting temperature.
In the preferred embodiment of the Benner et al. method, a reagent is added to maintain the melting point of the ash at a substantially definite point below the flame temperature. When this is accomplished, a thin layer of solid or very viscous ash is reportedly built up on the inside walls of the furnace. Additional ash that may accumulate during combustion remains in a liquid state and does not form deposits on the furnace walls. This liquid ash collects in the bottom of the furnace where it may be readily removed. A second embodiment of the method of Benner et al. operates to increase the melting point of the ash to a point above the flame temperature in order that the ash may be removed as a powder.
In addition to the efforts that have been made to alleviate the environmental problems associated with the presence of sulfur and ash in carbonaceous materials such as coal, work has been carried out to reduce the sulfur content of metallurgical coke which is produced from coal. One such method for producing low-sulfur coke from coal is described in U.S. Pat. No. 1,545,620 of Trent. According to this method, coal is finely pulverized and then saturated with water. Powdered limestone (calcium carbonate) is mixed with hydrocarbon oil and this mixture is added to the water-saturated or -suspended coal particles to form a plastic mass in which there is a close association between the sulfur and the limestone. Trent teaches that when this mass is coked, some of the sulfur is separated from the coal as sulfur gases, but the predominant portion thereof chemically combines with the limestone and forms calcium sulfide in the coke.
A method for treating coke which contains sulfur in order to reduce the chance that the sulfur will unite with the metal produced in a blast furnace in which the coke is used is described in U.S. Pat. No. 1,007,153 of Spurrier. According to this method, sodium carbonate or a salt, hydrate or oxide of one of the alkali metals is dissolved in water and the solution is poured over the coke. The water is then permitted to evaporate. As taught by Spurrier, this leaves the alkali compound in the pores of the coke where it may react with the sulfur upon combustion to form sulfates and sulfides of the alkali metal.
It is apparent from the foregoing that the addition of various additives to carbonaceous materials during or prior to their heating or combustion is known for the purposes of reducing or controlling the liberation of sulfur compounds and ash during the heating or combustion. It is also known that certain advantages inhere in the handling of carbonaceous materials in the form of a waterbased slurry. For example, some such slurries may be pumped like a liquid fuel. In addition, water-based coal slurries may be handled without dust problems and may be stored without the fear of spontaneous combustion.
Various methods for the burning of coal slurries are also known. For example, U.S. Pat. No. 3,313,251 of Jonakin describes a method by which a pipeline coal slurry may be mechanically dewatered and burned in a cyclone furnace. According to this method, coal slurries containing 50-70% crushed coal and the remainder water may be pumped through a pipeline to a dewatering centrifuge, where a portion of the water is removed, leaving a slurry containing about 20% water. This low-water slurry is admitted to a cyclone furnace for combustion. The temperature in the furnace is maintained above the melting temperature of the ash in the coal so that a molten ash residue is produced by the combustion process. The centrifugal action produced in the cyclone causes this residue to impinge on the furnace walls; however, under the influence of gravity, it flows to the bottom of the furnace where it may be removed.
Another method for burning a carbonaceous material such as coal in the form of a water-based slurry is described in my co-pending U.S. Patent Application Ser. No. 47,004, now U.S. Pat. No. 4,232,615, entitled "Coal Burning Method to Reduce Particulate and Sulfur Emissions". According to this method, a slurry is formed which contains the carbonaceous material, water and a reagent adapted to reach during combustion with the sulfur. The slurry may also contain a particulate binding agent adapted to reduce particulate emissions during combustion. The slurry is burned in a first stage with less than 100% theoretical air and preferably at a temperature below about 1100.degree. C. The products of combustion from this first stage are then removed to a second stage and burned with additional air.
The method of the aforementioned application provides for reduced emission of ash particulates and sulfur compounds in the combustion gases and for ease of handling of the fuel by virtue of its pumpable, slurry form. However, it has been found, in the practice of this method, that at relatively high velocities of combustion air into the first stage combustion chamber, such as about 0.3 meters/sec., solid particles in the fuel slurry may become entrained in the air and may be emitted as particulates in the exhaust from the second stage combustion chamber.